Evaluating the Effect of Some Commonly Used Idealizations on the Nonlinear Seismic Response of Steel Buildings

Abstract

There have been significant advances concerning seismic analysis and design procedures for steel buildings. Emerging procedures as the performance-based seismic design and the resilient seismic design, are examples of such advances. However, there is a long road ahead before these new concepts can be widely accepted and implemented by structural engineers, government officials and owners. In addition, many simplifications are commonly adopted in seismic analysis, the effects of which on the seismic response, and consequently on the structural performance, are overlooked. Some of these issues are addressed in this paper. Toward this goal, the seismic responses of 3 models of moment-resisting steel frames subject to 20 strong motion records are estimated. Results indicate that the ductility reduction factors for SDOF systems can be up to 95% greater than those of MDOF systems. Bending moments on columns and interstory shears can be underestimated by up to about 30% and 15%, respectively, when using the lumped mass matrix formulation. Underestimates of up to 18% are observed for bending moments on beams if just one element is used to model the beams. Overestimates can be up to 210% for lateral damping forces when using the first two modes of lateral vibration to generate the Rayleigh damping matrix; underestimations of up to 40% are observed for axial loads and flexural moments, while those of lateral shears and drifts can be of up to 20%. The contribution of local rotations and vertical displacements modes to axial loads on columns can be up to 38%, while for bending moments, interstory shears and drifts it can be up to 18%. A damping ratio of 3% to express the yielding effect is very conservative; a value of 7% can be reasonably used. The above results clearly indicate that more accurate structural models (MASM) should be used. One of the justifications for not using more accurate structural models (MDOF systems, consistent mass matrix and the Superposition of the Modal Damping Matrices to derive the damping matrix) has been the high demand on computing time; using modern computers, however, this justification is no longer valid.